Fluid catalytic cracking with supplemental heat

ABSTRACT

A potentially heat-deficient fluid catalytic cracking process for effecting a bulk boiling point conversion of a high boiling point petroleum feed to lower boiling products in which the overall enthalpy balance between the endothermic cracking and exothermic regeneration is maintained by combustion of a supplemental fuel in the middle or upper region of the dense bed in the regenerator (including the region immediately above the dense phase bed). There is a direct economic benefit from operation of the cracking process in this way since the preferred supplemental fuel is methane (natural gas) which is currently a low cost fuel while liquid products are higher value. Use of natural gas as a supplemental fuel will allow re-optimization of the catalyst and operations separately from the heat balance demand.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 62/172,917 filed Jun. 9, 2015, which is herein incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the fluid catalytic cracking (FCC)process for converting high boiling point petroleum oils to lowerboiling products.

BACKGROUND OF THE INVENTION

The fluid catalytic cracking (FCC) process has become the pre-eminentsource for motor gasoline in the USA and also serves the petrochemicalindustry with light olefins as petrochemical feedstock. In the FCCprocess, a pre-heated high boiling point petroleum feed such as a vacuumgas oil or residual fraction is subjected to a bulk boiling rangeconversion by contact with a hot, acidic-function catalyst in aspecialized process unit in which the feed comes into contact with thehot catalyst at the bottom of a tall vertical pipe or “riser” in whichthe essential cracking reactions take place to produce a range ofcracked hydrocarbon fragments in the vapor phase. The mixture ofcatalyst, vaporous cracking products and unconverted residues thenenters a disengaging zone in which the catalyst is separated from thehydrocarbons, usually by cyclones or other inertial devices; for reasonsarising from the early history of the process, the disengaging zone isusually referred to as the “reactor” although the majority of thecracking reactions take place, as intended, in the riser and theintention is that cracking in the reactor itself should be minimized.The separated, spent catalyst is then stripped of occluded hydrocarbonswith steam in a stripping zone at the bottom of the reactor and thestripped catalyst is sent to a regenerator in which the carbon (‘coke)which accumulates on the catalyst as a result of the carbon rejectionreactions taking place during the cracking process is oxidativelycombusted to reactivate the catalyst and to supply the heat for theendothermic cracking reactions. The hot catalyst from the regenerator isthen recirculated to the riser to participate in another round ofcracking.

Many types of FCC unit exist with variations too numerous to describedhere but all rest upon a few simple principles; namely, that thecracking reactions which effectively reduce the boiling point range andmolecular weight of the hydrocarbon species in the feed, are endothermicin nature are mediated by the acidic functioning catalyst which becomesdeactivated as a result of the endothermic cracking by the deposition ofrejected carbon onto the catalyst. This carbon (“coke”) is then burnedoff in the regenerator to supply the heat required for the crackingreactions to take place and this heat is carried from the regenerator tothe cracking riser by the circulating catalyst. In this way, an overallheat balance for the unit is maintained with only a minor portion of thetotal heat requirement being provided by the feed pre-heat. Thus, theoperation of the unit depends on the balance between the heat consumedby the endothermic cracking reactions and the exothermic combustion ofthe coke on the spent catalyst.

For FCCs processing all but the heaviest feed (highest Conradson Carboncontent), some of the coke which is burned in the regenerator is‘discretionary’, to the extent that it is burned in the regenerator tosupply the heat required for stable, heat-balanced operation. Withlighter (lower boiling) feeds and hydroprocessed feeds, stable unitoperation becomes problematical without operating under conditions whichresult in a greater amount of coke being generated during the crackingportion of the FCC cycle simply in order to supply the heat demands ofthe unit. Thus, economically valuable hydrocarbon liquids are turned tolow value coke merely to sustain operations.

U.S. Pat. No. 8,753,502 (Sexton) describes a way to maintain the overallunit enthalpy balance by combusting a low carbon fuel in a FCC catalystheater-fuel gas/catalyst combustion chamber of specialized designthrough which the catalyst is circulated. While noting that mostconventional FCC feedstocks contain enough coke precursors in the formof multi-ring aromatics to deposit sufficient “catalytic coke” on thecirculating catalyst to satisfy the overall unit enthalpy balance whileachieving the desired level of conversion, it is also noted that FCCprocesses have continued to evolve with unit designs that offer greaterprocessing flexibility with enhanced product yields via improved cokeselectivity, i.e. less coke relative to liquid product volume. Catalystimprovements have also enabled coke yields to be reduced but in allcases, the unit enthalpy balance must be met via a certain amount ofcoke or coke yield on fresh feed regardless of the feedstock's quality.Thus, changes in catalyst, unit design and/or operations could be madeto decrease the in-unit coke yield with consequent increases in liquidproduct yield but the heat balance then registers a deficit.

U.S. Pat. No. 8,354,065 (Sexton) presents a related conceptual approachusing a combination of the FCC catalyst heater-fuel gas/catalystcombustion chamber with a catalyst cooler.

While the proposals in U.S. Pat. Nos. 8,753,502 and 8,354,065 appear inprinciple of maintaining the unit enthalpy balance, they fail to makethe most effective use of the existing unit, requiring specialized unitmodifications to provide the combustion chambers in which lighthydrocarbon fuels are burned to supply the additional heat with thisheat being transferred to the main body of circulating catalyst by meansof a catalyst stream which is passed into or through the combustionchamber. With combustion chambers such as those described, necessarilyoperating at a high temperature, there exists the potential for catalysthold-up in the chamber, excessive erosion by fast circulating catalystflows, as well as inefficient heat transfer by a limited stream ofcatalyst.

SUMMARY OF THE INVENTION

We have now devised an improved scheme for redressing an inadequate heatsupply to the cracking reactions in the FCC unit by the substitution ofmethane (natural gas) (or other light fuels such as fuel gas) for this‘discretionary coke’. The combustion of this supplementary fuel in theregenerator itself will result in increased regenerator heat release forthe same amount of air/oxygen consumption while decreasing CO₂emissions. The supplementary fuel is injected into a dense bed of thecatalyst in the middle or upper region of the dense bed or even or justabove the bed. Operation of the regenerator in this way will maximizethe oxygen concentration at the bottom of regenerator for coke burningpurposes which is a slower reaction while essentially eliminating excessoxygen in the dilute phase and the flue gas resulting in a more reducingatmosphere for minimizing NOx formation. The added heat of combustion isdirectly added to the body of catalyst being regenerated so avoidingproblems of heat transfer and catalyst hang-ups in separate combustionchambers.

According to the present invention, a potentially heat-deficient fluidcatalytic cracking process is modified to maintain an overall enthalpybalance between the endothermic cracking and exothermic regeneration bycombustion of a supplemental fuel in the middle or upper region of thedense bed in the regenerator (including the region immediately above thedense phase bed).

The present fluid catalytic cracking process for effecting a bulkboiling point conversion of a high boiling point petroleum feed to lowerboiling products contacts the feed with a hot cracking catalyst toeffect endothermic cracking of the feed after which the spent catalystis exothermically regenerated by oxidative combustion of coke depositedon the catalyst during the cracking in a dense bed of catalyst in aregeneration step; in this process, the overall enthalpy balance betweenthe endothermic cracking and exothermic regeneration is maintained bycombustion of a supplemental fuel in the middle or upper region of theregeneration dense bed (including the region immediately above the densephase bed). There is a direct economic benefit from operation of thecracking process in this way since the preferred supplemental fuel ismethane (natural gas) which is currently a low cost fuel in the USAwhile liquid products are higher value. Use of natural gas as asupplemental fuel (in a manner similar to torch oil in the regenerator)will allow re-optimization of the catalyst and operations separatelyfrom the heat balance demand.

DRAWINGS

The single FIGURE of the accompanying drawings is a simplified diagramof an FCC regenerator modified for the supply of supplemental heat bycombustion of a light fuel in the regenerator.

DETAILED DESCRIPTION

With the current economic conditions (primarily in the USA) natural gasis a significantly economically advantaged fuel, while incremental‘discretionary coke’ generated and burned in an FCC regenerator ispotential liquid product of higher value. In addition, increasedpolitical pressure on decreasing greenhouse gas emissions drives towardusing higher hydrogen content fuels; methane is the highest hydrogencontent fuel. Methane releases about 7% more heat per unit of airconsumed than does a typical FCC coke (at ˜7% hydrogen in coke); it doesthat while producing about 40% less CO₂ per unit quantity of heatreleased. Burning methane to heat balance the unit, instead of thenormal practice of burning incremental ‘discretionary coke’, allowsrecovery of that ‘discretionary coke’ as liquid products while reducingunit emissions not only of CO₂ but also of NOx and SOx from theregenerator. The substitution of higher hydrogen content material forindigenous coke increases the heat release per unit of air, henceincreasing the FCCs processing capacity.

While methane is the preferred fuel, similar although less markedbenefits may be secured by using a light (low carbon) fuel such asrefinery fuel gas, (mostly carbon monoxide), hydrogen, syngas or evenlight hydrocarbons such as ethane or propane if available in sufficientquantity and economically justifiable.

For full burn FCCs operating on light or hydrotreated feed, it isestimated that 20-50% of the coke currently burned in the regeneratormay be ‘discretionary coke’, for which natural gas could be substituted.In a 100 KBD (about 16,000 m³/day) FCC this could result in anadditional 1-2+ KBD (about 160-320+m³/day) of liquid product yield and350-700 ktons/yr (315-630 ktonnes/yr) (˜10-20%) reduction in CO₂emissions, at the expense of an equivalent (heating value basis) amountof methane.

The FIGURE is a simplified fragmentary section of an FCC regenerator ofthe dense bed type. The regenerator comprises a body 10 linked to thecracking section (not shown) of the FCC unit in the normal way by meansof a catalyst standpipe 11 for the transfer of spent, stripped catalystfrom the stripper section of the reactor to the base of the regeneratoras well as a returned catalyst standpipe linked to the foot of thecracking riser (not shown) via the conventional slide valve forcontrolling catalyst flow rate. The spend, coked catalyst is regeneratedin a bed 12 in the regenerator by oxidative combustion of the coke onthe catalyst in the presence of air or oxygen-enriched air fromdistributor 15. The bed has the characteristics of a dense bed 16 in thebase region of the regenerator body immediately above the distributorand in this region the oxygen concentration relative to coke is high asa result of being in the vicinity of the oxygen-bearing gas entering thebed from distributor 15. The average bed density progressively decreaseswith ascending height in the bed which eventually becomes a dilute phase17 higher up in the regenerator. The gases from the regeneration processleave the body of the regenerator by way of cyclone system 18 comprisingboth primary and secondary cyclones which return separated catalystparticles to the catalyst bed through the catalyst diplegs in the normalway. The off-gases exit the regenerator through plenum 19 and pass asstack gases to precipitators, filters and baghouse as is conventional.

Natural gas or other light (low carbon) supplemental fuel is injectedinto the middle region of the bed by means of a series of fuel injectorsin the middle level (one only shown) 22 of the catalyst bed or at theupper level 22 (one only shown) either into the dense bed or where thedense phase catalyst particles enter the dilute phase; the amount ofoxygen at this point is still adequate to ensure combustion of thesupplemental fuel so that heat is generated for heat balance to maintainoperation of the unit with an adequately high returned catalysttemperature. The injectors will normally be arranged uniformly aroundthe periphery of the reactor to promote even heating or, if the catalystcirculation pattern in the regenerator is known to be non-uniform, inconformity with the established pattern to promote uniform heat transferto the mass of catalyst according to the local bed density in theregenerator at the level at which the injectors are located. Theinjectors will be selected to provide a flow rate according to thesupplemental fuel in use and its heating value.

The catalyst particles absorb heat from the combustion of the fuel aswell as from the combustion of the coke on the particles themselves andcarry it to the cracking reactions via the returned catalyst standpipe.The relative amounts of the fuel and the oxygen content of the catalystbed at the level of fuel injection should be adjusted to ensure thatthere is sufficient oxygen in the regeneration gas for completecombustion of the added fuel and that the amount of added fuel issufficient to supply the required amount of supplemental heat. As notedabove, addition of the supplementary into the dense bed in the middle orupper region of the dense bed or even or just above the bed maximizesthe oxygen concentration at the bottom of the regenerator so thatcombustion of the coke on the catalyst is maximized to form aregenerated catalyst with a desirably low residual carbon content; thecarbon burning process is a slower reaction and therefore has time to beessentially complete by the time the catalyst has passed through thedepth of the bed; at the same time, injection of the supplemental fuelat a higher level in the bed essentially eliminates excess oxygen in thedilute phase and the flue gas resulting in a more reducing atmospherefor minimizing NOx formation.

The present process is primarily applicable in those cases wherecracking of the selected feed could be carried out to better advantagewith regard to liquid yield by changes in operation or selection ofcatalyst but where such changes have so far been implemented because ofthe need to maintain unit heat balance. The addition of the heat fromthe supplemental fuel enables this to be done with a consequentialimprovement in liquid product yield and a reduction in CO₂ and othergaseous emissions. Feeds to the unit will therefore tend to be lighter(lower boiling) distillate feeds such as gas oils with an end pointtypically below 550° C. (about 1020° F.), hydroprocessed feeds or mixedfeeds in which feeds of this kind will predominate. Cracking conditionswill be as appropriate for the selected feed and catalyst and the unitunder consideration.

What is claimed is:
 1. In a fluid catalytic cracking process foreffecting a bulk boiling point conversion of a high boiling pointpetroleum feed to lower boiling products in which the feed is contactedwith a hot cracking catalyst to effect endothermic cracking of the feedafter which the spent catalyst is exothermically regenerated byoxidative combustion of coke deposited on the catalyst during thecracking in a fluidized regeneration bed of catalyst in a regenerationstep, the improvement comprising combusting a supplemental fuel in themiddle or upper region of the regeneration bed or the region immediatelyabove the bed.
 2. A fluid catalytic cracking process according to claim1 in which the combustion of the supplemental fuel maintains an overallenthalpy balance between the endothermic cracking and exothermicregeneration.
 3. A fluid catalytic cracking process according to claim 1in which the supplemental fuel is injected into the middle region of theregeneration bed at a level at which the amount of oxygen in the bed isadequate to ensure combustion of the supplemental fuel.
 4. A fluidcatalytic cracking process according to claim 1 in which thesupplemental fuel is injected into the dense bed in the upper region ofthe regeneration bed at a level at which the amount of oxygen in the bedis adequate to ensure combustion of the supplemental fuel.
 5. A fluidcatalytic cracking process according to claim 1 in which thesupplemental fuel is injected into the dense bed at a level wherecatalyst particles enter a dilute phase above the dense bed and at whichthe amount of oxygen in the bed is adequate to ensure combustion of thesupplemental fuel.
 6. A fluid catalytic cracking process according toclaim 1 in which the supplemental fuel comprises a gaseous fuel.
 7. Afluid catalytic cracking process according to claim 1 in which thesupplemental fuel comprises natural gas.
 8. A fluid catalytic crackingprocess according to claim 1 in which the supplemental fuel comprisessyngas.
 9. A fluid catalytic cracking process according to claim 1 inwhich the high boiling point petroleum feed comprises a distillate feed.10. A fluid catalytic cracking process according to claim 1 in which thehigh boiling point petroleum feed comprises a distillate feed having anend point not higher than 550° C.